In recent years, an IP telephone system has been popularized. In the IP telephone system, video and audio are transmitted and received in the form of packet data bi-directionally in real time via an IP network, such as a Local Area Network (LAN). In the IP telephone system, a call control server and a plurality of IP telephone terminals are connected to an IP network and communication between IP telephone terminals or communication between an IP telephone terminal and an outside line can be performed on a call control server basis.
In the IP telephone system, when communication is performed, a session is established between an IP telephone terminal on the calling party side and that on the called party side using such a protocol as SIP under the control of a call control server. After the session has been established, audio communication is performed by peer-to-peer connection which omits a switching process performed by a call control server. In a peer-to-peer connection between IP telephone terminals, audio packets are exchanged between individual IP telephone terminals using a common audio media codec (e.g., G.711, G.722, or G.729).
In the IP telephone system, a specific holding tone is produced so that a caller may hear the tone when the caller is put on hold or a specific tone is made during the call to let the caller hear the tone. In addition, the IP telephone system provides various services using a device called a media server when one-to-many communication or many-to-many communication is performed in a meeting or paging.
A telephone system in which a sound source server that holds a holding tone sends a holding tone packet to a gateway via unicast transmission and the gateway converts the holding tone packet into a unicast one and transmits it to an outside line has been proposed as related technology (e.g., Jpn. Pat. Appln. KOKAI Publication 2008-147887).
The media server can be realized using hardware or software. When the media server is realized using hardware, audio processing requires special hardware. Since the number of simultaneous conversions depends on the number of conversion circuits, offering services simultaneously in response to many calls results in an increase in cost.
When the media server is realized using software, there is a virtual upper limit to the number of simultaneous conversions according to the processing capability of the CPU. To provide services simultaneously in response to many calls, it is necessary to use a high-performance, expensive CPU or provide a plurality of devices on which media server software runs, increasing costs. In addition, since the maximum number of ports used by media servers has already been specified in the contract when media servers are installed, only as many ports as up to the upper limit in the contract can be used even if there are empty ports.